I use both programs, but I don't think you can make sensible use of LinkBack between them. If you copy text from NWP and paste it in iCalamus, then double click the text frame, you just get to edit the text in the frame in iCalamus. In a way that makes sense, because you would want to be able to edit the text directly in iCalamus to see how e.g. insertion or deletion of text changes your layout. On the other hand, it would be great if you could commit changes made in iCalamus back to your RTF file. That functionality is currently not implemented in iCalamus.
Otherwise, the two pieces of software seem to be a promising combo. Though both rely on the OS X text engine and therefore suffer from some of the same shortcomings, most notably neither offers soft hyphens. These shortcomings will likely stay around in both programs until Apple fixes them. There might be other issues, like footnotes (I don't know if/how import of those works in iCalamus), but these might be solved in future versions. After all both are 1.0s. Some might point to Nisus' express history, but iCalamus it definitely a 1.0 (which unfortunately shows in some annoying bugs), plus there is a plugin architecture so we may hope for add-ons.